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Every day in the ED we see patients with vague, nonspecific abdominal pain, and in a lot of EDs a clean-catch urinalysis is part of these patients’ standard lab workup. And in a lot of ED’s there is great variation in the way different providers interpret those urinalyses and decide who gets diagnosed with, and treated for, a UTI. So what does the evidence say about this? Not as much as you’d hope – despite being such a common laboratory test, data on how good urinalysis is at predicting UTI (defined by symptoms plus a positive urine culture) is surprisingly sparse and inconsistent. Hence all the practice variation. But after an extensive literature review, here are the best evidence-based answers I could produce to some questions we face every day in the ED.

Note that this literature review applies only to female adult patients. Laboratory definition of a “positive urine culture” varies a little between labs and studies, but usually is defined as a culture that grows out >104-105 colony forming units of a single organism. Definition of a “contaminated culture” similarly varies, but is usually one that grows out multiple organisms, or <104-105 of a single organism.

How many squamous cells constitute a “contaminated” sample that isn’t worth interpreting?The idea that presence of squamous cells on microscopy means the urine sample was contaminated by skin flora is a very old, very widespread concept. Believe it or not, there is only one set of published data exploring it1, and that data found the following:

The confidence intervals are so big because this was a small study (105 patients), but it’s literally the only data we’ve got. And based on it, while the complete absence of squamous cells may well rule out contamination, the presence of any squamous cell at all means the possibility (but far from the certainty) of contamination. Conclusion: Squamous cells probably matter, but we don’t know how much, nor where the useful cutoff is. For practical purposes, you can think of zero squamous cells meaning no contamination. Any squamous cells means potential contamination. Does the “clean catch” method (wiping with a disinfectant wipe, then collecting a midstream sample) work to prevent contamination?There are quite a few studies and resulting editorials saying that the clean catch method simply does not consistently prevent urine sample contamination.2-10 My favorite of the studies is from 2015, in which 40 completely healthy, asymptomatic female ED staff (nurses, PA’s, residents) gave clean catch samples, and also samples in which they just peed in a cup with no clean catch methodology.6 The results were as follows:

In other words, even ED staff members, the people you would think most educated on the clean catch method, all trying as hard as they could, couldn’t get their contamination rate below 63%.

Conclusion: No, it probably doesn’t work. Be very cautious in using any data from a non-catheterized urine sample.

Is a catheterized sample any better?As with the last question, the data here is fairly robust.2-10 As a representative example, here is another table from that study evaluating whether squamous cells on microscopy predict contamination.1

Conclusion: Yes, cath urine is probably better.What findings should constitute a “positive” urinalysis?This is one of the hardest questions to answer, because it has been extensively studied but the results are extremely inconsistent.11-16, 21 While there are certainly associations between leukocyte esterase, blood, RBC, WBC and presence of bacteria with positive urine culture, the literature is currently unable to provide any consistent cutoff above which you can say the patient definitely has a UTI. One table from a meta-analysis trying to answer the question is shown here16:​

With the exception of >5 WBC on microscopy being quite sensitive (though very nonspecific) and presence of nitrites and bacteria on microscopy being quite specific (though very nonsensitive), those ranges are all over the place. The finding that nitrites predict a positive urine culture actually plays out pretty well, with specificity never less than 92% in any study I have cited. The numbers for all other dipstick and microscopy findings are much less consistent across the cited studies. Conclusion: Presence of nitrites probably does predict positive urine culture with some certainty. Absence of >5WBC on microscopy probably makes it much less likely that the culture will be positive. For all other dipstick and microscopy criteria, associations certainly exist, but the literature cannot give us consistent, evidence-based cutoffs for how to most effectively use them.What symptoms differentiate asymptomatic bacteruria from UTI?Remember that just because the urinalysis is positive doesn’t mean the patient has a urinary tract infection – it just means they have a significant level of bacteria in their urine, which in the absence of symptoms is called “asymptomatic bacteruria.” And, according to the most recent guidelines by IDSA, AAFP, USPTF and ACOG, the only patients who need antibiotics for asymptomatic bacteruria are pregnant women.22-25 Studies are currently underway to determine if renal transplant patients should be added to this list; for now, practice varies widely.26 It is well documented in the literature that physicians overtreat asymptomatic bacteruria, mistaking it for UTI despite a lack of symptoms. 17Several authors have tried to answer this question, and in doing so create a clinical prediction tool for UTI. 16-21 Their results vary a little, but not much. In a representative study, a meta-analysis from 2002 published in JAMA, the symptoms statistically likely to predict uncomplicated UTI were19:Dysuria (Pos LR 1.5)Urinary Frequency (Pos LR 1.8)Hematuria [reported by the patient, not measured on a urinalysis] (Pos LR 2.0)Absence of vaginal discharge/irritation (Neg LR 3.1/2.7)It’s worth noting that lower abdominal pain did NOT come out as a significant predictor (Pos LR 0.9-1.4). The common sense (though not explicitly studied) corollary of this would be that vague, nonspecific abdominal pain is probably no better a predictor, and there were no findings in any study I could find to contradict that conclusion. Conclusion: Presence of dysuria, frequency and patient-reported hematuria predict urinary tract infection. Absence of vaginal discharge/irritation also predicts urinary tract infection. Generalized, nonspecific abdominal pain is most likely NOT indicative of urinary tract infection, and a positive urinalysis in one of these patients is likely asymptomatic bacteruria.

You are doing a great job! Your article is very professional and I really appreiate it.

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